Characterization of Silver Nanoparticles under Environmentally Relevant Conditions Using Asymmetrical Flow Field-Flow Fractionation (AF4)

Abstract

The development of methods to monitor manufactured nanomaterials in the environment is one of the crucial areas for the assessment of their risk. More specifically, particle size analysis is a key element, because many properties of nanomaterial are size dependent. The sizing of nanomaterials in real environments is challenging due to their heterogeneity and reactivity with other environmental components. In this study, the fractionation and characterization of a mixture of polyvinylpyrrolidone-coated silver nanoparticles (PVP-AgNPs) of three different sizes were investigated using asymmetrical flow field-flow fractionation (AF4) coupled with UV-Vis spectrophotometry. In particular, the effects of electrolyte composition and natural organic matter (NOM) on the particle size and stability were evaluated. The fractogram peaks (i.e., stability) of three different AgNPs decreased in the presence of both 10 mM NaCl and 10 mM CaCl2, while increased with increasing concentration of humic acid (HA). In addition, the hydrodynamic diameters of AgNPs in both electrolytes slightly increased with an increase of HA concentration, suggesting the adsorption (coating) of HA onto the particle surface. It is also interesting to note that an increase in the particle size depended on the types of electrolyte, which could be explained by the conformational characteristics of the adsorbed HA layers. Consistent these results, AgNPs suspended in lake water containing relatively high concentration of organic carbon (TOC) showed higher particle stability and larger particle size (i.e., by approximately 4 nm) than those in river water. In conclusion, the application of AF4 coupled with highly sensitive detectors could be a powerful method to characterize nanoparticles in natural waters.

Fig 1. TEM image (A and B) and particle size distribution determined by batch-DLS (C) for AgNPs mixture.

Fig 2. AF4 fractogram of a AgNPs mixture (A) and TEM images of fractions collected at elution time of 13 (B), 23 (C), and 36 min (D), respectively.

Fig 3. AF4 fractograms of AgNPs mixture suspended in various types of electrolytes.

Fig 4. NOM effect; AF4 fractograms (A) and zeta potential (B) of AgNPs mixture suspended in 10 mM NaCl.

Fig 5. NOM effect; AF4 fractograms (A) and zeta potential (B) of AgNPs mixture suspended in 10 mM CaCl₂.

Fig 6. Particle size distribution determined by batch-DLS (A) and AF4 fractograms (B) of AgNPs mixture suspended in natural water.